Universal joint



Sept 2, 1952 R. CHILTON l2,608,839

UNIVERSAL JOINT INVENTCR RCILAND EHILTUN ATTORNEY Sept 2, 1952 R. CHILTON 2,608,839

UNIVERSAL JOINT Filed Mal-C1123, 1948 2 SHEETS-SHEET 2 94 2 a4 ma 'la 9o, 106 4,@

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66 I: 1 E l INVENTOR RDLAND CHILTDN.

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ATTO RN EY Patented Sept. 2, 1952 UNIVERSAL JOINT Roland Chilton, Glen Rock, N. J., assignor to Curtiss-Wright Corporation, a corporation of j lDelaware Application March 23, 1948, Serial No. 16,450

11 Claims.

Thisinvention relates to a flexible couplin for a pair of shafts and is particularly directed to a torsionally rigid flexible coupling having three degrees of freedom such that said coupling permits lateral, axial and angular displacement of said shafts relative to each other.

An object of the invention comprises the provision of a novel flexible coupling of compact construction and having the aforementioned three degrees of freedom. A further object of the invention comprises a flexible coupling having said three degrees of freedom and being capable of transmitting large torques, `-as for a tank transmission. Although the invention has been designed for use with a tank transmission it obviously is of general application.

Specifically, the invention comprises a pair of spherical-ended struts disposed on diametrically opposite sides of the coupling axis. Each of said spherical-ended struts is disposed between a pair of nat parallel surfaces respectively rigid with one of the shafts to be coupled together and with an intermediate coupling member, torque being transmitted inone direction from said one shaft to said intermediate member through said surfaces and the interposed spherical-ended struts. Torque is similarly transmitted from said intermediate member to the other of the-shafts to be coupled together through a second pair of diametrically spaced spherical-ended struts, ro`V tatively displaced 90 from said first pair of spherical members. Two additional pairs of spherical-ended struts are Aprovided for transmitting torque in the reverse direction.

Other objects of the invention will become apparent upon reading the' annexed detailed description in connection with the drawing,- in which:

Figure l is a diagrammatic perspective view of a coupling illustrating the mode of operation of the invention;v I

Figure 2 is an exploded perspective view of a coupling embodying the. invention; f

Figures 3 and 4 are sectionalviews taken along 1ines 2-2 and 3-3 respectively of Figure 2 but with the coupling completely assembled; and- Figure 5 is a partial view of a modification of the coupling of Figures 2 to 4.

The mode of operation of applicants coupling can best be understood by reference to the diagrammatic showing of Figure 1. This gure illustrates a coupling providing a flexible connection between a pair of shafts lllA and I2. In Figure 1, the coupling comprises la pair of identical flat U-shaped or forkedmembers I4 and i5 connected to the'shafts lil and l2, respectively. The iiat U-shaped members i4 'and I6 are rotatively displaced relative to each other with their respective flat projecting ends I8 and 2B extending in opposite directions and overlapping an intermediate coupling member 22. The intermediate coupling member 22 is formed 'with corner portions 24, 26, 28 and 30 having spaces therebetween withinV which the projecting ends I8 and 20 are disposed. `One ofl-sphericalballs 32, 34, 36, 38, 59,42, M, and E is disposed be tween each said corner portion and the adjacent projecting flat end I8 or 20. The facing surfaces of said corner portions and the projecting ends IS are nat and disposed parallel to the axis of the intermediate coupling member 22. The facing surfaces of said corner portions andthe projecting ends 20 are also flat and disposed parallel to the axis of the intermediate coupling member 2S, said latter facing surfaces being disposed at right angles t0 said rstmentioned facingr surfaces.

YWith this arrangement of Figure 1, the ecupling is adapted to'transmit clockwise torque from the shaft l0 to the shaft coupling member i4 and thence through the balls 38 and 46 to the intermediate'coupling member 22. From the intermediate coupling member 22 said clockwise torque is transmitted through the 'balls 32 ande@ to the shaft coupling member I6 and thence to the shaft I2. Similarly, the coupling is adapted to transmit counter-clockwise,torque4 from the shaft l0 to the shaft coupling member i4 and thence through the balls 35' and M to-'the intermediatecoupling member 22. `From the intermediate coupling member 22 saidk counter-clockwise torque is transmitted through the' balls 34 and 42 tothe shaft coupling member` t6 and thence to the shaft l 2.k Thus, one set of four balls 32, 38, 40, and 45 With the intermediate coupling member 22 transmits torque in one direction and the other set of four balls 34.136,' 42,'andi4with said intermediate ycoupling member transmits torque inthe other direction. Accordingly, if torque is only to be transmitted in one'direction, then one of said sets of balls could be-eliminated. 'From yFigure 1 it is clearthat, because ofthe rolling action of the spherical ballsv on the 'hat surfaces engaged thereby, the shaft lcoupling member I4 may move axially and laterallyhor izontally relative to the intermediate member -22 and in addition vsaid coupling member rlll may rotate about a vertical axis Y'-Y relative to the intermediate coupling member 22. -Similarly'the Shaftcoupling member I'Elh'i'ay move axially and aco'sse laterally vertically relative to the intermediate coupling member 22 as well as rotate about a horizontal axis X-X relative to said intermediate coupling member. Accordingly, the shaft coupling members I4 and I6 are free to move axially, laterally and angularly relative to each other. Such flexing movements of the coupling members result only from the rolling action of the spherical balls on the fiat surfaces engaged thereby, the two facing flat surfaces engaged by each spherical :ball remaining parallel to each other during all flexing movements of the coupling. Thus, there is no sliding friction between any of the relatively moving parts ofthe coupling. The magnitude of the relative movements between the coupling members I4, I6 and 22 is obviously limited by the amount of clearance provided between said coupling members. Also, it should be clear from Figure 1, that the coupling is torsionally rigid, that is, that the coupling does not permit relative rotative movement of the shafts IU and l2 relative to each other about their axis.

With the arrangement of Figure l, the smaller the spherical balls are made, the higher are the f contact stresses on the surfaces engaged by said balls. Accordingly it is desirable that the spherical ballsbe as large as possible. However, in Figure 1, the overall diameter of the coupling is many times that of its spherical balls, so that the l size of these spherical balls is severely limited by the permissible size of the coupling.

Figures 2 to 4 illustrate a coupling generally similar to that of Figure 1 in which a novel arrangement permits the use of torque transmitting spherical surfaces of relatively large radius.

With the arrangement of Figure 1, the torque transmitting spherical surfaces are formed on complete spheres or balls. However, because of the limited relative movement of the coupling parts, it is only necessary that each spherical surface have a rolling action of a few degrees in any one direction. Accordingly it is possible, in Figure 1, to eliminate all of the surface of' each spherical ball except that small portion of each spherical surface engageable with the coupling members I4, I6, or 22. That is,'each spherical ball of Figure 1 may be replaced by a strut having spherical end surfaces, the two end surfaces of each strut forming part of a common sphere. 'Such a structure is illustrated in Figures 2 to 4.

Referring now to Figures 2 to 4, there is illustrated a flexible coupling comprising forked coupling members 52 and 54.' The coupling members 52 and 54 are rotationally rigid with shafts 56 and 58 respectively, which shafts are to be coupled together. The shaft coupling members 52 and 54 are flexibly connected together through a rigid intermediate coupling member 68, said latter member being disposed between the shaft coupling members 52 and 54. The shaft coupling member 52 is provided with a pair of prong-like projecting portions 62 diametrically spaced and axially extending relative to the axis of its shaft. In addition, the shaft coupling member 54 is provided with a pair of diametrically spaced and axially extending projecting portions 64 similar to the projecting portions 62. The projecting portions 62 and 64 are received in the spaces between the corner portions 66 of the intermediate coupling member' 60 with the coupling members 62 and 64 disposed on opposite sides of said intermediate coupling member and with the projecting portions 62 of the shaft coupling member 52 rotatively displaced 90 relative 4 to the projecting portions 64 of the shaft coupling member 54.

Each projecting portion 62 of the shaft coupling member 52 is provided with a pair of spaced parallel bores or recesses 68 and 10 extending therethrough. The axes of one pair of bores 68 and 'l0 define a plane parallel to the corresponding plane defined by the other pair of bores or recesses 68 and 10, said planes being disposed on opposite sides of the coupling axis. The bores 68 and 'l0 are counterbored to form shoulders 12. Each bore 68 is counterbored from an end opposite to the end from which its adjacent bore 'l0 is counterbored and opposite to the end from which the other bore 68 is counterbored. A cup-shaped member 74 is received within each bore 68 and '10, said cup-shaped members having annular external flanges 16 engaging the shoulders 'l2 in their respective bores 68 or l0. The bottom interior surface of each cup-shaped member 'I4 is flat and is disposed at right angles to the axis of its bore. With this arrangement the fiat bottom surfaces of the cup-shaped members 'I4 are parallel to each other and said bottom surface for each bore 68 faces ina direction opposite to the bottom surface for its adjacent bore 'l0 and opposite tothe bottom surface for the other bore 68.

The shaft coupling member 54 is identical to that of the shaft couplingrmember 52, said shaft coupling members being rotatively displaced 90 relative to each other. Thus, each projecting portion 64 of the coupling member 54 is provided with a pair of parallel cylindrical bores or recesses 'I8 and 80 having cup-shaped members 82 closing opposite ends of the bores of each said pair. As illustrated, the cup-shaped members 'i4 and 82 are similar and the interior bottom surface of the cup-shaped members 82 comprises parallel flat surfaces disposed at right angles to the flat bottom surfaces of the cup-shaped members 14.

The corner portions 66 of the intermediate coupling member 60 are provided with cylindrical bores or recesses 86 and 88 parallel to and facing the open ends of the bores 68 and 'I0 respectively, in the projecting portions 62 of the shaft coupling member 52. In addition said corner portions are provided with cylindrical bores or recesses 90 and 92 parallel to and facing the bores 18 and 86 respectively, in the projecting portions 64 of the shaft coupling member |54. The ends of the bores 86, 88, and 92 remote` from the open ends of their respective facing bores 68, '10, 'I8 and 80, are closed by. cup-shaped members 94 threadedly secured therein. The interior bottom surface of the cup-shaped members 04 are flat and are disposed parallel to the flat bottom interior surfaces of their respective facing cupshaped members 14 and 82. Elongate sphericalended struts 96, 98, l00, and |02 are disposed between said parallel flat surfaces as follows: There are two struts 96, each of which is disposed between the facing parallel flat bottom surfaces of a pair of bores 68 and 86. Similarly there aretwo struts'98, each disposed between the facing parallel' flat bottom surfaces of a pair of bores 10 and 88. In addition there are two struts 100, each of said struts being disposed between the facing parallel flat bottom surfaces of a pair of bores 18 and 90, and likewise there are `twostruts |02, each of said latter struts being disposed between the facing parallel flat bottom surfaces of a'. pair of bores 80 and 92.

The struts 92,V 98, |00, and 162 are identical and' the end surface |04 of each strut is spherical with .the center of each spherical surface being disportions 62 and 64 between the -cornerportions SS of the intermediate coupling member 60 and to permit the coupling to be adjusted to zero backlash, one of the cup-shaped members 9 4 for the two .struts 96 is screwed down against ashoulder |96 while the cup-shaped member 94 for the other strut 96 is adjusted down againstv said ,strut. Similarly one cup-shaped member 94 for each of the struts 98, |00, and |02 is adjusted 'down against a shoulder |09 While the other cup-shaped members 94 for said struts are adjusted down against their respective struts. In thisway the projecting portions. E2 and 54 can be accurately located between the corner portions `t of Athe intermediate coupling member 50 with the ends of each strut 96, 98, |00, and |02y disposed in engagement with the bottoms of their associated recesses with zero backlash therebetween. With thisl construction and as viewed in the drawing, the spherical ended struts 96 and 00 prevent counter-clockwise and clockwise rotation, respectively, of the shaft coupling member 52 relative to the intermediate coupling .member 60.

`Similarly, the spherical ended struts |00 and |02 prevent clockwise and counter-clockwise rotation, respectively; of the shaft coupling member 54 relative to the intermediate coupling member 50. Accordingly, the coupling is torsionally rigid.

As illustrated, the cross-sectional diameter of the struts 95, 90, |00, and |02'is a .maximum at their spherical end surfaces, said maximum diameter being substantially equal Ato the internal diameter -of the cup-'shaped members within which the struts are received. Thus each strut tapers' to its 'minimum diameter 'mid-section thereby providing clearance between' said'struts and the side walls of their recesses or bores Within which said struts are received; This clearance permits limited rolling movement'of the spherical ends of each strut on the flat surfaces'engaged thereby. In addition, clearance is'provided between the projecting portions 02 and 54 and the intermediate coupling member Ellas best seen in Figures 3 and 4.- Accordingly, as viewed in Figure 2, the rolling action of the spherical ends of the struts 96 and 98 permitsaxialinovement, lateral horizontal movement, and rotative movement about a vertical axis, of the shaft coupling member 52 relative to the coupling member 80. Similarly, as viewed in Figure 2, the rollT ing action of the spherical ends of the 'struts- |00 and |02 permits axial and lateral horizontal movement, as well as rotativev movement about a horizontal axis, of the shaft coupling member 5 4 relative to the intermediate coupling member 60. Because of the rolling action of the spherical end surfaces of each strut with the flat parallel surfaces engaged thereby, the clearances, between the shaft coupling portions 62 and 64 andthe intermediate coupling member corner portions 58, remain unchanged as the 'couplingflexes f Therefore, said clearances can be madeffquite small. Also the pair of facing flat couplingsurfl faces engaged by thespherical ends of eachstrut remain parallel to each other as thecoupling ilexes;

It should new be clear that the structure of Figures `2 te 4 provides. the same degrees of freedom between the `shafts 56 and 58 Vas providedby the coupling of Figure l between the shafts I0 and l2.' However, in the structure ofv Figuresv 2 to 4, the-spherical end surface fof each strut forms part of a sphere having a diameter not much vsmaller than the diameter of thecoupling itself as,y compared to vthe relatively small spheres. of Figure 1. As previously mentioned, increasing ythe diameter of vthe spherical coupling surfaces reduces the magnitude of the contact stresses on the -surfaces engaged thereby. The large diameter of .the spherical surfaces in Figures 2te 4 is made possible by the .overlapping and the staggered :arrangement of each pair of struts 5:6.and 58 and each pair of struts '60 and 62, and'by the fact that said struts extend into recesses in the coupling .members'52, 54, and 60. In addition to providing the coupling with spherical surfaces of large diameter, the large depthof the recesses 58, 10, 18, and 80 provides the coupling projecting portions 52 and 64 with a larg'ebeam depth and therefore vwith considerable strength.

Since al1 the torque' transmittedfrom the shaft 56 to theshaft 58 and vice versa, is transmitted .through `the spherical e'nd surfacesv |04 of the struts 96, 98', |00, and |02, `.andthrough the ilat surfaces engaged thereby, it is important that there be no sliding -between said surfaces. That is, it is importantthat both spherical ends of each said strut be constrained-to rolling action on the flat surfaces engaged thereby.' For vthis purpose,

' both ends of each strut 98, 98, |00, |02v and the cooperating surfaces inthe recesses within which said struts are received, may bemodified, from that illustrated in Figures 2 to 4, to prevent relative sliding of their engaged torquetransmitting surfaces. Figure 5 illustrates a'construction of one end of a strut and its cooperating recess to prevent said relative sliding. `The coupling of Figure 5 is like that of Figures 2'tofl except each end of `a strut and its recess is modified as illustrated in Figure 5.

InFigure 5, a cupshaped member |I0 is Yprovided with a atbottomsurface ||2 and with an internal conical side wall A| I4 disposed about said flat bottom surface. One end of an elongate strut H8 extends into vvsaid cup-shaped member, f

there being a similar construction at the other end of said strut. Each strut H6 is also provided with an external annular surface |20 engaging the conical surface ||4 of'its cup-shaped member ||0. j

In Figure 5 as in Figures 2 to f1, the two spherical end'surfaces I8 of a strutl I5 comprise part of a common sphere whereby the radius of each said spherical surface isequal to one-half the length of its strut. In addition, in Figure V5 as in Figures 2 to 4, the couplingtorque is transmitted through vthe spherical end surfaces ||8 of the struts and the flat surfaces" I |2 engaged thereby. Thus, the coupling of Figure5'is videntical with that of Figures 2 to 'except eachI of the struts 98, 98, |00, and |02 of Figures 2 to 4 isfreplaced by a strut H6 and eachcup-shaped member'14, 82, and 94 of Figures'2 to `4 y'is replacedby a cup-shaped member I0.' Accordingly the cou` pling ofY Figure5 includes all the features of the coupling of Figures 2 to 4 and inlfaddition each torque transmitting strut' i's" provided with an annular surface y|20 adjacent-to each of its ends. In addition,tliebottom ofl each strut receiving recess is provided with an ,annular surface ||4 Ven" gageableby the annular surface |20 on the end 7 of thestrut received within said recess in order to prevent sliding between the associated spherical surface H8 and the flat surface ||2 engaged thereby. Y l

The engaging annular surfaces I I4 and |20 are such that in an axial plane therethrough, their profiles comprise that of meshing rack and pinion teeth respectively, with the pitch circle of the pinion tooth lying in the spherical end surface I I8 of its strut I I6 and with the pitch line ofthe rack lying in the fiat bottom surface H2 of its member I I0. In Figure' the surfaces I I4 and |20 have beenV continued by dot and dash lines |22 and |24 respectively, to complete their tooth profiles only for the purpose of more clearly bringing out therack and pinion tooth nature of said profiles. Involute rack and pinion teeth have been illustrated in Figure 5. Obviously, however, other rack and ,pinion tooth profiles may be used.

With the coupling construction ofFigure 5, as a spherical end surface H8 of a strut H6 rolls on the flat surface H2 engaged thereby, the surfaces H4 and |20 remain in engagement, said surfaces H4 and |20 moving relative to each other as meshing rack and pinion teeth with the pitch circle of the pinion tooth lying in said spherical end surface I|8. Therefore, no sliding can take place between the spherical end surface H8 of each strut and the flat surface I l2 engaged thereby, the engaging annular surfaces H4 and |20 constraining said spherical end surface H8 to rolling action of the fiat bottom H2 of its recess. y

The contact stresses between the annular surfaces H4 and |20 result only from theleffect of centrifugal forces on the struts H6, all the coupling torque being transmitted through the spherical ends H8 of the struts HB and the fiat surfaces H2 engaged thereby. Accordingly, the contact stresses between the annular surfaces I4 and |20 are small.

As described, the two torque transmitting spherical end surfaces of each strut of Figures 2 to 4 and of Figure 5 have a common center with a radiusequal to one-half the length of the strut. However, the radius of each said spherical end surface may be'made somewhat larger than one-half the length of the strut with the center of each said spherical surface disposed on the longitudinal axis of the strut. Then, when the coupling flexes from its uri-flexed condition, the rolling action of the struts results in an increase in their effective length thereby elastically loading said struts ,and providing a slight restoring force tending to return the coupling members to their un-exed condition. Y

While I have described my invention in detail in its present preferred embodiments, it will be obvious to those skilled inthe art, after understanding my invention, that various changcs and modifications may be made therein without departing from the spirit or scope thereof- I aim in the appended claims to cover all-such modifications.v s Y l I claim as myinvention: 1

l. A flexible coupling for a-pair of shafts: said coupling comprising first and'second members each adapted to be rotationally rigid with one of said shafts," said first and second members each having a pair of -diametricallyspaced portions on opposite sides of .the couplingaxis, said pairs of diametrically spaced portions, being` rotatively displaced 90 relative-to each other; each of said s said strut overlapping the strut extending into the other recess in the associated one of said portions of said first and second members.

2. A flexible coupling comprising four elongate struts each having a convex spherical surface adjacent to its two ends, said struts being so disposed and having a length such that they substantially form the sidesof a square about the coupling axis; first and second coupling members and an intermediate coupling member having projections each containing a pocket providing a fiat contact face disposed near the corners of said square, each said contact face engaging an end of one of said struts.

3. A flexible coupling for a pair of shafts comprising a first coupling member arranged for connection to one of said shafts; a second coupling member arranged for connection tothe other of said shafts, each of said coupling members having a pair of diametrically spaced projections rotaitively displaced 90 relative to the pair of projections of the other coupling member, each of said projections having a pair of oppositely facing parallel overlapping recesses disposed at right angles to the coupling axis and to the diameter on which the projections of its coupling member are disposed; a third coupling member having four projections symmetrically spaced about its axis and between which said first and second member projections are received, said third member projections having recesses facing the recesses in said first and second member projections; each of said recesses extending inwardly from one side of its projection and having a closed bottom adjacent to the opposite side of said projection with said closed bottom including a flat surface disposed parallel to the corresponding flat surface of the facing recess; and a plurality of elongate struts, one for each pair of facing recesses and extending into said recesses, each of said struts having a convex spherical surface adjacent to each of its ends engaging the fiat bottom surfaces of the recesses within which said strut is received.

4. A flexible coupling for a pair of shafts comprising a first coupling member arranged for connection to one of said shafts; a second coupling member arranged for connection to the other of said shafts, each said coupling member having a pair of diametrically spaced projections rotatively displaced 90 relative to the pair of projections of the other coupling member and each of said projections having a pair of oppositely-facing laterally-spaced parallel overlapping recesses disposed at right angles to the coupling axis and to the diameter on which the projections of its coupling member are disposed; a third coupling member having four projections symmetrically spaced about its axis and between which said first and second member projections are recel-ved, said third member projections having recesses facing the recesses in said first and second member projections; each of said recesses in said members having a flat surface adjacent to its bottom disposed parallel to the corresponding flat surface portions having a pair :of oppositelyj facing of its facing recess; and a plurality of e10ngate struts. one for each pair of facing recesses and extending into said recesses, each of said struts having a convex spherical surface adjacent to each of its ends engaging the fiat bottom surfaces of the recesses within which said strut is received.

5. A flexible coupling for a pair of shafts comprising a Afirst coupling member arranged for connection to one of said shafts; a second coupling member arranged for connection to the other of said shafts, each said coupling member having a pair of diametrically spaced projections rotatively displaced 90 relative to the pair of projections of the other coupling member and each of said projections having a pair of oppositely-facing parallel overlapping recesses axially spaced relative to the coupling axis and disposed at right angles to said axis and to the diameter on which the projections of its coupling member are disposed; a third coupling member having four projections symmetrically spaced about its axis and between which said first and second member projections are received, said third member projections having recesses facing the recesses in said first and second member projections; each of said recesses in said members having a fiat surface adjacent to its bottom disposed parallel to the corresponding flat surface of its facing recess; and a plurality of elongate struts, one for each pair of facing recesses and extending into said recesses, each of said struts having a convex spherical surface adjacent to each of its ends engaging the fiat bottom surfaces of the recesses within which said strut is received.

6. A flexible coupling as recited in claim 3 in which each strut and i-ts pocket have engaging annular surfaces with the profile of one of said surfaces in a plane axial relative to said surfaces being similar to the truncated conical profile of a rack tooth and the corresponding profile of the other of said surfaces being similar to that of a pinion tooth meshing with said rack tooth.

'7. A flexible coupling comprising first and second coupling members each having a pair of diametrically spaced projections with each said pair of projections being rotatively displaced 90 relative to the other pair of said projections, each of said projections having a pair of oppositely-facing laterally-spaced parallel overlapping recesses disposed at right angles to the coupling axis and to the diameter on which the projections of its coupling member are disposed; a third coupling member having four projections symmetrically spaced about its axis and between which said rst and second member projections are received,

said third member projections having recesses o facing the recesses in said rst and second member projections, each of said recesses having a fiat surface disposed adjacent to its bottom and disposed parallel to the corresponding flat surface of its facing recess; and a plurality o-f elongate struts, one for each pair of facing recesses and extending into said recesses, each of said struts having a convex spherical surface at each end engaging the at bottom surfaces of the recesses within which said strut is received.

8. A flexible coupling as recited in claim 7 in which the pair of oppositely facing recesses Vof said rst and second member projections overlap to such an extent that the struts extending into said recesses overlap each other along at least half their lengths.

9. A flexible coupling as recited in claim '7 in which the radius of each convex spherical end surface of a strut is at least equal to one-half the distance between convex end surfaces of said strut.

10. A exible coupling as recited in claim 7 in which the radius of each convex spherical end surface of a strut is larger than one-half the distance between the convex end surfaces of said strut.

11. A flexible coupling comprising first and second coupling members having a pair of diametrically spaced projections with each said pair of projections being rotatively displaced relative to the other pair of said projections, each of said projections having a pair of rst and second oppositely-facing parallel recesses with the first recesses of each pair being disposed in a plane transverse to the coupling axis and axially spaced along said axis from the corresponding plane of the second recesses of each pair; a third coupling member having four projections symmetrically spaced about said axis and between which said first and second member projections are received, said third member projections having recesses facing the recesses in the first and second member projections, each of said first, second and third member recesses having a flat surface disposed adjacent to its bottom and disposed parallel to the corresponding flat surface of its facing recess; and first and second sets of four elongate struts respectively disposed in said rst and second planes with the four struts of each set being disposed about said axis so as substantially to form the sides of a square, each said strut having a convex spherical surface at its two ends and extending into a recess in one of said first and second members and into the facing recess of said third member With the spherical ends of said strut engaging the flat bottom surfaces of said recesses.

ROLAND CHILTON.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,493,232 Barton May 6, 1924 1,499,618 Kasley July 1, 1924 1,648,078 Thomsen Nov. 8, 192'7 1,734,043 Nelson Nov. 5, 1929 FOREIGN PATENTS Number Country Date 321,077 Great Britain 1929 

